Origin of the Unusual Composition of ³He-rich Solar Energetic Particles

Abstract We examine 3 He-rich solar energetic particles (SEPs) detected on 2023 October 24–25 by Solar Orbiter at 0.47 au. Measurements reveal that heavy-ion enhancements increase irregularly with mass, peaking at S. C, and especially N, Si, and S, stand out in the enhancement pattern with large abundances. Except for 3 He, heavy-ion spectra can only be measured below 0.5 MeV nucleon −1 . At 0.386 MeV nucleon −1 , the event shows a huge 3 He/ 4 He ratio of 75.2 ± 33.9, larger than previously observed. Solar Dynamics Observatory extreme-ultraviolet data show a minifilament eruption at the solar source of 3 He-rich SEPs that triggered a straight tiny jet. Located at the boundary of a low-latitude coronal hole, the jet base is a bright, small-scale region with a supergranulation scale size. The emission measure (EM) provides relatively cold source temperatures of 1.5–1.7 MK between the filament eruption and nonthermal type III radio burst onset. Analysis suggests that the EM distribution of temperature in the solar source could be a factor that affects the preferential selection of heavy ions for heating or acceleration, thus shaping the observed enhancement pattern. Including previously reported similar events indicates that the eruption of the minifilament is a common feature of events with heavy-ion enhancement not ordered by mass. Surprisingly, sources with weak magnetic fields show extreme 3 He enrichment in these events. Moreover, the energy attained by heavy ions seems to be influenced by the size and form of the jets.